as my experience playing staffM, valkyrie couldn't replace exo in a staffM's form, since staffM's AP
regen is very poor.
imo your form is good enough. you can replace general with valkyrie when facing stun form but i still prefer general.
how's valkyrie recruitment there? is it event or there's a questline?
if it's event just recruit her and level her to at least lvl 121.
her skill books and upgrade is free with the event and i see no reason to not recruiting her.
afaik staffM is pretty decent even after riederan's invasion patch. but if you're really aiming for PvP
you should change to spearM later.
a mandatory mercenary you should recruit later is Roro Kidul.
Attila & Mwindo isn't bad but they're not a must.